To understand why, NPR conducted a poll with the Robert Wood Johnson Foundation and the Harvard School of Public Health. It focuses on what happens in American households during the hours between school and bedtime.

This is crunch time for most families — when crucial everyday decisions get made about food and exercise.

Our poll used a unique design to get at what is actually happening in the life of a "target child" in each household. We supplemented their responses with more than 800 that came in when we asked parents, through NPR's Facebook page, to describe their own "crunch times."

The most striking finding is that U.S. parents "get it."

When we asked a parent or other principal caregiver in our poll how important it is that their child eats and exercises in a way to maintain a healthy weight, more than 9 in 10 said it was important — and most said it was "very important."

But all too often, there's a disconnect. Despite good intentions, it's not happening.

More than half of children ate or drank something during the "crunch time" window that can lead to unhealthy weight gain, as perceived by their parents. And more than a quarter of children did not get enough exercise, their parents say.

"It's hard enough to get dinner on the table while trying to help them with homework," says Paige Pavlik of Raleigh, N.C. "Once we do everything, there is absolutely no time to go outside and take a walk or get any exercise. It's simply come in, eat, sit down, do homework, go to bed."

The relentlessness of it makes her emotional. Pavlik starts to cry as she talked about her family's daily crunch time. "It's really hard," she says. "This isn't how I thought family life was going to be."

Lori Bishop of Lexington, Ky., says she tries to excel as a parent and as a full-time laboratory manager. But she ends up feeling mediocre at both.

Both she and her husband have stressful jobs, she says, and often feel exhausted at the end of the workday. "But you gotta go right to work in the kitchen," Bishop says. "And while I would love to prepare a well-balanced meal each evening ... it doesn't happen."

She says dinner at the Bishops' house is often pre-packaged meals — "things that are frozen that you can heat up like frozen pizza, frozen chicken nuggets."

In three-quarters of the households polled, most of the family ate dinner together the previous evening. And of those, most said the "target" child's dinner was prepared at home with fresh ingredients.

But about a third of children who eat at home with their families are like Lori Bishop's kids — they end up relying on pre-packaged, frozen or take-out food.

And nearly half of those in our poll say it's difficult for families to eat together on a daily basis.

Adam Jacobs' family in Mesa, Ariz., is a case in point. He and his wife have long commutes to work. "My wife and I don't even get to talk about our days," he says. "If I have something to tell her, I literally put it on my smart phone to remind me."

Their two boys, who are 14 and 10, usually have after-school activities. On one recent evening, Jacobs tried to rustle up dinner for himself and his older son.

"I was at home at 7:30 and it was ready by 8:30," he says. "And by that time, my wife and younger boy weren't even home yet and my older son had already eaten ... so it wound up just being another solo deal."

It's not just time that's in short supply. Among the parents we talked to who say it's difficult to prepare fresh foods, money was also a factor.

Araceli Flores and her two young children live in the farm belt of central California, surrounded by fields of broccoli, cauliflower, asparagus and fruit orchards. But she says it's too expensive to buy fresh foods.

"I can buy a box of macaroni and cheese for a dollar," Flores says. "A bunch of bananas – like a good maybe week-and-a-half's worth of bananas — will cost me over a dollar. Strawberries are four dollars. Apples, a bag of apples, is going to cost me five dollars — way more pricier to buy vegetables and fruits than it is to buy boxed food."

And it's not just time and money that get in the way of doing the right thing. Parents say there are other barriers — such as their physical surroundings, school lunch policies, and their family's culture.

Vanessa Benavides is a single mom in Miami who's Cuban-American. She comes from a family with nutrition-related health issues.

"Most of my family is overweight," she says. "Most of my family has high blood pressure. My father's diabetic."

On weekends, Benavides fills up her grocery cart with fresh fruits and vegetables. She intends to cook healthy meals, she says. But at the end of a long day when she picks up 7-year-old Emily at her mom's house, it's often easier to eat the dinner that her mother has made.

"It's a lot of Cuban meals," Benavides says. "You know, it's just a lot of rice-meat combinations that are often fried or greasy."

And then Benavides' mom and dad keep urging Emily to eat more. "They want to fatten her up," she says.

That bothers Benavides, because she was once a skinny kid like Emily. But now she struggles with her weight.

"They were always trying to tell me to eat more and eat more and eat more," she says. "You know, a nice chunky kid is [considered] healthy in our culture."

Parents often say schools are undermining the messages many parents are trying to give their children. In school cafeterias across the country, kids are still eating dense, high-fat and over-processed food for lunch.

Assonta Wagner, one of our Facebook respondents, lives with her husband and four children in Alamogordo, N.M. Her kids typically take their lunch to school. But sometimes she allows them to eat school lunches. And when they do, she cringes.

"One of their favorite meals was this mashed potato bowl," Wagner says. "It was mashed potatoes, little fried chicken pieces and corn. And then they covered it in gravy."

She's a stay-at-home mom who's super-conscientious about making healthy food at home — lots of fruits, vegetables and lean meats, no pre-packaged foods, no instant meals, no soda. So what's offered at school is particularly upsetting to her.

And it's not just the lunches. At Halloween, Thanksgiving and other holidays, they get bags of candy — even her overweight son — as a reward for doing a good job.

And when all those calories add up over the years, parents say the very environment they live in makes it hard to reverse the problem. Some parents say it's too hard — or too expensive — to get their children to a safe place to exercise.

That's one reason one poll respondent, Vivian Carter-Smith, is so worried about her 17-year-old grandson.

"He's a good boy," she says. "He's overweight. He's short ... and I know he weighs 350 pounds easy."

She says her grandson doesn't get any exercise. He stopped going to the "Y" when other boys teased him about his weight. And Carter-Smith says there isn't a place near his Cincinnati home where her grandson and his younger brother can even get outside and walk. And in fact, they don't want to.

"The way the streets are," she says, "with these kids with no parents and no rules and regulations and upbringing skills — they are scared."

But she knows that a lack of exercise combined with an unhealthy diet can lead to disaster. After all, her grandson's father died last year of a heart attack – at the age of 38.

Today in Your Health, we're starting a series on why so many American children are overweight or obese. That subject led us to look at the hours between school and bedtime, crunch time for most families when crucial decisions get made about food and exercise.

We conducted a poll with the Robert Wood Johnson Foundation and the Harvard School of Public Health. And the poll finds parents understand what kids need to do. The challenge is doing it.

Here are NPR health correspondents Richard Knox and Patti Neighmond.

PATTI NEIGHMOND, BYLINE: The most striking thing about our poll is that nearly everybody wants to make sure their kids eat right and exercise enough. But a lot of the time it's not happening.

RICHARD KNOX, BYLINE: Paige Pavlik of Raleigh, North Carolina, says the relentless checklist of things that have to be crammed between afterschool and bedtime make every day a struggle.

PAIGE PAVLIK: It's hard enough to get dinner on the table while trying to help them with homework. Once we do everything there is absolutely no time to go outside and take a walk or get any exercise. It's simply come in, eat, sit down, do homework, go to bed.

KNOX: Pavlik says she burst into tears when she saw our invitation on NPR's Facebook page asking parents to describe their crunch times.

PAVLIK: And it makes me cry now. It just, it's really hard. This isn't how I thought family life was going to be.

NEIGHMOND: Lori Bishop of Lexington, Kentucky, has similar feelings. She tries to excel both as a parent and on the job as a full-time laboratory manager. But she says she ends up feeling mediocre at both.

LORI BISHOP: My husband and I don't have physically demanding jobs so we're not physically tired when we get home. But both of our jobs are very stressful. So I find that we are very taxed by the time we get home, which makes us very exhausted. But you got to go right to work in the kitchen. So, while I would love to prepare a well-balanced meal each evening, it doesn't happen.

NEIGHMOND: And when it does happen, more often than not, its convenience foods.

BISHOP: We eat a lot of pre-packaged meals; things that are frozen that you can just heat up, like frozen pizza, frozen chicken nuggets.

KNOX: Many parents in our poll say their children eat meals prepared at home with fresh ingredients. But about a third of children who eat at home with their families are like Lori Bishop's kids, they end up relying on pre-packaged, frozen or take-out food.

NEIGHMOND: And when it comes to sitting around the table for dinner virtually everybody in our poll says it's important. Nutritionists say families who eat together are healthier. But nearly half of those in our poll say that's difficult.

KNOX: Like Adam Jacobs. Both he and his wife have long commutes to work from their home in Mesa, Arizona.

ADAM JACOBS: My wife and I don't even get to talk about our days. You know, if I have something to tell her, I literally put it on my smartphone to remind me.

KNOX: Their two boys, 14 and 10, have activities that keep them after school. So, here's an example of what typically happens. On one recent evening, Jacobs tried to rustle up a dinner for himself and his older son.

JACOBS: I was home at 7:30 and it was ready by 8:30. And by that time, my wife and younger boy weren't even home yet and my older son had already eaten. And because they were out, they also ate out - my wife and my younger boy. They just bought something while they were out. So it wound up just being another solo deal.

(LAUGHTER)

JACOBS: So, just sit and wait.

NEIGHMOND: And it's not just time that's in short supply. Among the parents we talked to, who say they find it difficult to prepare fresh foods, money was also a factor.

Araceli Flores and her two young children live in the farm belt of Central California, surrounded by fields of broccoli, cauliflower, asparagus and fruits. But buying fresh foods she says is just too expensive.

ARACELI FLORES: I can buy a box of macaroni and cheese for a dollar and a bunch of bananas. Like, a good maybe week and a half worth of bananas will cost me over a dollar. Strawberries are $4. A bag of apples is going to cost me $5. I mean, way more pricier to buy vegetables and fruit than it is to buy boxed food.

KNOX: Time and money aren't the only things that get in the way of doing the right thing. Parents say there are other barriers; their physical surroundings, school lunch policies and their family culture.

Vanessa Benavides is a single mom in Miami who's Cuban-American. She comes from a family with nutrition-related health issues.

VANESSA BENAVIDES: Most of my family is overweight. You know, most of my family has high blood pressure. My father is diabetic.

KNOX: On the weekend, Benavides fills up her grocery cart with fresh fruits and vegetables. She intends to cook healthy meals. But at the end of a long day, when she picks up 7-year-old Emily at her mom's house, it's often easier to eat the dinner her Mom has made.

BENAVIDES: It's a lot of Cuban meals. You know, it's just a lot of rice-meat combinations that are often fried or greasy.

KNOX: And then her mom and dad urge Emily to eat more.

BENAVIDES: They want to fatten her up. She needs to eat more. She needs to eat more.

KNOX: That bothers Benavides because she was once a skinny kid like Emily. But now she struggles with her weight.

BENAVIDES: They were always trying to tell me to eat and eat more and eat more and eat more. And, you know, a nice chunky kid is healthy in our culture.

NEIGHMOND: And even schools are undermining the messages that many parents are trying to give their children. In school cafeterias across the country kids are still eating dense, high-fat and over-processed lunches.

Assonta Wagner lives with her husband and four children in Alamogordo, New Mexico. Her kids typically bring their lunch to school. But sometimes she allows them to eat the school lunches and when they do she cringes.

ASSONTA WAGNER: One of their favorite meals was this mashed potato bowl, where it was mashed potatoes and little fried chicken pieces and corn, and then they covered it in gravy. And she's like, oh, everybody really loves that dish.

NEIGHMOND: Wagner's a stay-at-home Mom - who's super-conscientious about making healthy food at home. Lots of fruits, vegetables and lean meats. No pre-packaged foods. No instant meals. No soda.

In fact, when we asked parents who dined at home the previous night what they ate, two-thirds said their kids had meals prepared from scratch with fresh ingredients. So what's offered at school is particularly upsetting. And it's not just - the lunches.

WAGNER: They come home with a bag of candy, you know, for every little thing.

NEIGHMOND: Halloween, Thanksgiving. One of her children is overweight and, even for a job well done at school, he gets a bag of candy.

WAGNER: Snickers and Hershey Kisses and lollipops, Starbursts and those little Halloween-sized piece of candies - just a little bag of those all tied up all nice and pretty.

(LAUGHTER)

WAGNER: Of course, by the time I got it, it was mostly nice and pretty little wrappers.

(LAUGHTER)

KNOX: And when all those calories add up over the years, the very environment can make it hard to reverse the problem. Some parents say it's too hard or too expensive to get their kids to a safe place to exercise.

NEIGHMOND: That's one reason one of our poll respondents, Vivian Carter-Smith, is so worried about her 17-year-old grandson.